GREGORY STOCK has written an enthusiastic book in support of
germ-line manipulations -- that is, making genetic modifications
to eggs, sperm and embryos that can be passed on to future generations.
Like previous explorations of the subject by the ethicist Joseph
F. Fletcher, the lawyer John Robertson and the biologist James
Watson, among others, ''Redesigning Humans: Our Inevitable Genetic
Future'' serves as an apologia for those scientists and physicians
who are already edging toward such work in a piecemeal fashion
in research labs and in vitro fertilization clinics around the
world. It advocates the wholesale adoption of genetic manipulations
with the purpose of finally taking control of human evolution.
This, the author writes, ''is the ultimate expression and realization
of our humanity.''

Because of the breadth of his scientific knowledge and his
considerable flair as a writer, Stock -- who heads the program
on medicine, technology and society at the School of Medicine
of the University of California, Los Angeles -- is a forceful
advocate. First and foremost, he says, such manipulations are
inevitable -- an assertion he makes not only in his subtitle
but repeatedly throughout his book. Stock may be correct, for
many factors are driving genetic experimentation forward, including
professional ambition, greed and the desperation of infertile
couples who will submit to just about anything in order to have
a child.

And it should be added that eugenic considerations have already
entered into this enterprise (even though Stock states at one
point that in the infertility field, no one ''cares about such
wild notions as human redesign''). Numerous practitioners of
in vitro fertilization, starting with Robert Edwards of Baby
Louise Brown fame, have discussed their goals for ''improving''
the species. Meanwhile, would-be parents readily buy into schemes
to improve birth outcomes -- as they have done at least since
the days of ancient Greece.

But to argue the inevitability of all this is to sidestep the
paramount issue, which is how to balance the desires of researchers
and of the potential consumers of laboratory technologies against
the good of society. After all, scientists, when asked, will
nearly always say that they would like to get on with their
work unhampered by any sort of regulation.

However, unfettered science has not historically shown itself
to be in the best interest of society, any more than unfettered
government, religion or business have. The fact that biomedical
tinkerings have brought benefits for some does not constitute
a sufficient reason for concluding that the practitioners of
the embryonic arts should have the right to pursue any and all
lines of experiment. Nor is it an adequate argument to say that
if banned, germ-line manipulations would simply move offshore
or go to the black market. That may be true. But if the fact
that people will seek to avoid the law were a reason not to
have a law, we would have no laws at all.

Stock's overarching claim is that germ-line modifications will
''write a new page in the history of life, allowing us to seize
control of our evolutionary future,'' an echo of the classic
eugenicist dream. New technologies will allow humans to make
fundamental alterations to their individual genetic compositions
and those of their children. The net effect, he says, will be
to draw ''reproduction into a highly selective social process
that is far more rapid and effective at spreading successful
genes than traditional sexual competition and mate selection.''
In the future, he claims, we will be ''much more than simply
human.''

But Stock's assumptions regarding population biology and evolution
are suspect. For example, he assures us that we needn't worry
about any impact that germ-line alterations might have on the
gene pool (like reducing genetic diversity) because the number
of altered children would be quite small. But if the net effect
of all those laboratory-engineered births is so negligible,
how then can there be the major evolutionary changes he predicts?
Moreover, while it is clearly possible to affect the genetic
circumstances of small, reproductively isolated populations,
most population geneticists would agree that the goal of bringing
about substantial changes in a pool of six billion humans (and
growing) is near to impossible, especially any time soon.

So the great collective enterprise Stock envisions will in
all likelihood be limited only to the small percentage of people
who can afford or gain access to these technologies. And then
the issue becomes what these bermenschen might do with the rest
of us.

STOCK frequently acknowledges problems with a particular line
of experimentation, or impediments in the way of some of his
predictions. But in each case, once he is done listing objections,
he proceeds to a bold proclamation that effectively moots them
all.

Nor do moral and ethical concerns slow him down for long. He
declares, ''If biological manipulation is indeed a slippery
slope, then we are already sliding down that slope now and may
as well enjoy the ride.'' The lesson he takes from history is
that eugenics itself wasn't wrongheaded, just the nationalistic,
totalitarian applications of it. ''Given Hitler's appalling
foray into racial purification,'' he writes, ''European sensitivities
are understandable, but they miss the bigger picture.''

For Stock, this bigger picture is clear. The future is a time
in which individuals will be able to go into a clinic and, through
a simple procedure, obtain embryos fitted with chromosomal modules
that will slow aging, eliminate disease and enhance personality,
temperament, intellect and beauty. It's a pleasant enough fantasy.
But even if evolution could be steered in a positive direction,
why presume that humans have the wisdom to do so? ''Redesigning
Humans'' is an act of both boosterism and reductionism. It admits
but then ignores the enormous complexity of biological systems;
it places biology firmly above social, ecological and economic
considerations; and it reduces concepts like success in life
to the purely physical, as if health and longevity were the
only issues that mattered. Isn't it pretty to think so?

Gina Maranto is the author of ''Quest for Perfection: The Drive
to Breed Better Human Beings.''

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